Chapter 3: Electromagnetism Form 5 Physics Next > The study of matter Chapter 3: Electromagnetism 1

Physics: Chapter 3 Objectives: (what you will learn) 1) magnetic effect of current-carrying conductor 2) force on current-carrying conductor in magnetic field 3) electromagnetic induction 4) transformers 5) generation & transmission of electricity < Back Next > 2

Line of Force A line of force in magnetic field represents path of free N-pole in magnetic field. Direction of line of force: N-pole S-pole Line of force Magnetic field around the Earth < Back Next > Magnetic field around a bar magnet http://motivate.maths.org/conferences/conf44/c44_talk4.shtml Magnetism and the Magnetic Compass http://pilotsweb.com/navigate/compass.htm A MAGNET IN SPACE http://stargazers.gsfc.nasa.gov/students/magnet_in_space_sp.htm Pilotsweb.com Stargazers 3

Magnetic effect When current flows in a conductor, a magnetic field is produced around it. Magnetic field can be observed by sprinkling iron filings around wire on a piece of cardboard. < Back Next > The direction of field can be obtained by moving a compass around the wire. 4

Magnetic effect The 2-dimensional view of magnetic field due to current in straight wire is easier to draw. Current up: Current coming out of paper Current down: Current going into paper < Back Next > As distance from wire increases, magnetic field gets weaker (as shown by increasing distance between lines). http://library.thinkquest.org/15433/unit6/6-4.htm 5

Magnetic effect Without compass, the direction of magnetic field can be obtained using Right-Hand Grip Rule. < Back Next > http://www.uq.edu.au/_School_Science_Lessons/UNPh30.html Right-Hand Grip Rule Grip wire with the right hand and with the thumb pointing in the direction of current. The other fingers point in the direction of magnetic field. 6

Solenoid Current, I in circular coil creates magnetic field where it is strongest along the axis. Solenoid is formed from many circular coils of wire uniformly wound in the shape of a cylinder through which electric current flows. < Back Next > http://web.singnet.com.sg/~hcting/ Magnetic field pattern produced by a current in a solenoid is almost identical to that of a bar magnet. The direction of the field, B is determined using right-hand grip rule (R.H.). 7

Solenoid Solenoids are important because they can create controlled magnetic fields and can be used as electromagnets. < Back Next > To find the N-pole of solenoid, grip it with right hand, the fingers curl in the direction of current, and the thumb points in the direction of N-pole. http://www.hk-phy.org/main_e.html 8

Solenoid 9 The magnetic field inside a solenoid is given by: B = µnI This slide for extra information only. Solenoid The magnetic field inside a solenoid is given by: B = µnI B = magnetic field magnitude (teslas) µ = magnetic permeability (henries/meter or newtons/ampere2) n = turns density (number of turns/meter) I = current (amperes) n = N / h N = number of turns h = length of solenoid (meters) < Back Next > http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/solenoid.html http://en.wikipedia.org/wiki/Permeability_(electromagnetism) µ = ku0 magnetic constant or permeability of free space, µ0 = 4π x 10-7 H/m k = relative permeability 9

Electromagnet An electromagnet is made by winding a coil of wire around a soft iron core, which loses its magnetism when the current is switched off, unlike steel which is magnetized permanently. < Back Next > http://en.wikipedia.org/wiki/Electromagnets In electromechanical devices, direct current is used to create strong magnetic field for drawing iron core or plunger into it, such as in switches and relays. 10

Electromagnet 11 The strength of the electromagnet increases significantly with the use of soft iron core (µ) when the number of turns per unit length of the coil is increased (n) < Back Next > when the current in the coil is increased (I) B = µnI where µ = ku0 µ0 = 4π x 10-7 H/m (or N/A2) k = relative permeability of iron is about 200, steel over 800 11 Electromagnets are used in electric bells, circuit breakers, electromagnetic relays, telephone earpieces, etc.

Magnetic force Force, F (Motion) Field, B Current, I The direction of the force F on the conductor can be obtained using Fleming’s left-hand motor rule. < Back Next > http://www.newton.dep.anl.gov/askasci/phy05/phy05062.htm 12

Electromagnetic induction Electromagnetic induction is the production of induced e.m.f. in conductor when there is relative motion between conductor and magnetic field. Faraday’s law of electromagnetic induction The e.m.f. induced in a conductor is directly proportional to the rate of change of magnetic flux through the conductor. < Back Next > An e.m.f. is induced if wire cuts across magnetic field. 13 No e.m.f. is induced if the wire moved parallel to magnetic field; the magnetic lines of forces are not cut by the wire.

Electromagnetic induction Force, F (Motion) Field, B Current, I The direction of e.m.f. induced or the induced current I can be obtained using Fleming’s right-hand dynamo rule. < Back Next > 14

Electromagnetic induction Lenz’s law The direction of the induced current produces an effect that opposes the change in the magnetic flux. An e.m.f. is induced in a solenoid when a magnet is moved into or out of solenoid. The direction of induced current is obtained using Lenz’s law. < Back Next > 15 Induced current produces N-pole to repel the N-pole of magnet

Transformers Transformer is an application of electromagnetic induction. It consists of a primary coil and a secondary coil wound on a soft iron core. < Back Next > 16 Transformer is used to step-up or step-down the voltage of an a.c. supply, depending on where the a.c. source is applied.

Generation of Electricity Many sources of energy are used to generate electricity, each with their own advantages and disadvantages. Examples: Hydro Potential energy of water in a dam converted to kinetic energy Natural gas, diesel, coal Used as fuel to heat water in boilers to produce steam Biomass Waste material used as fuel, or decomposition of waste for methane gas for use as fuel. Nuclear energy Nuclear fission of uranium releases heat used to heat water. Sunlight Solar cells convert sunlight into electricity. Wind Strong wind rotates windmill-like blades to rotate turbines. < Back Next > 17

Generation of Electricity Many sources of energy are used to generate electricity, each with their own advantages and disadvantages. Examples: Hydro Potential energy of water in a dam converted to kinetic energy Natural gas, diesel, coal Used as fuel to heat water in boilers to produce steam Biomass Waste material used as fuel, or decomposition of waste for methane gas for use as fuel. Nuclear energy Nuclear fission of uranium releases heat used to heat water. Sunlight Solar cells convert sunlight into electricity. Wind Strong wind rotates windmill-like blades to rotate turbines. < Back Next > 18

Transmission of Electricity Alternating voltage is generated at power station as its voltage can be transformed with transformers. A step-up transformer changes voltage to 320 kV or 500 kV. < Back Next > Transmission at high voltage reduces current in cables; thus reducing power loss greatly. Power loss as heat in cables = I2R 19

Transmission of Electricity Voltage is stepped down in stages to, say 240 V using transformers before supplying to consumers. < Back Next > The National grid network is an interconnection of various power stations in the country. It ensures: minimal disruption to power supply through fast backups efficient power generation by matching demand with supply that power stations can shut down for regular maintenance 20

Summary 21 What you have learned: Thank You magnetic effect of current-carrying conductor 2. force on current-carrying conductor in magnetic field < Back 3. electromagnetic induction 4. transformers http://www.bbc.co.uk/scotland/education/bitesize/higher/physics/radiation/index.shtml 5. generation & transmission of electricity 21 Thank You